Thermally Activated Delayed Fluorescence Polymers Research Articles

Fluorene-carbazole-based hyperbranched polymers with linear red TADF chromophores for high-efficiency white and red electroluminescent devices

Solution-processed polymer light emitting devices (PLEDs) using high-quality thermally activated delayed fluorescence (TADF) polymers as the emitting layers are attractive, however, high-efficiency white-light TADF polymers are very rare because of the complicated radiative transition channels. In this study, a series of hyperbranched TADF conjugated polymers were designed and synthesized which adopted the linear D-A-type TADF emitter of 1,8-naphthalimide-acridine as red chromophores, conjugated poly(fluorene-carbazole) as the blue backbone and triphenylamine as the hyperbranched cores. By adjusting the appropriate feed ratios of monomers, energy transfer process from the blue backbone to the red chromophores could be regulated and HR0.8 film had the highest photoluminescent quantum yield of 24%. Consequently, its doped electroluminescent device showed the cold-white emission with the CIE color coordinates of (0.35, 0.23) and the maximum EQE of 6.82%. By simply raising the concentration of the red chromophore in the polymers, the rapid reverse intersystem crossing (RISC) process and the enlarged energy gap between S1 and T1 were achievable.

Acceptor Copolymerized Axially Chiral Conjugated Polymers with TADF Properties for Efficient Circularly Polarized Electroluminescence.

Chiral conjugated polymer has promoted the development of the efficient circularly polarized electroluminescence (CPEL) device, nevertheless, it remains a challenge to develop chiral polymers with high electroluminescence performance. Herein, by the acceptor copolymerization of axially chiral biphenyl emitting skeleton and benzophenone, a pair of axially chiral conjugated polymers namely R-PAC and S-PAC are synthesized. The target polymers exhibit obvious thermally activated delayed fluorescence (TADF) activities with high photoluminescence quantum yieldsof 81%. Moreover, the chiral polymers display significant circularly polarized luminescence features, with luminescence dissymmetry factor (|glum|) of nearly 3 × 10-3. By using the chiral polymers as emitters, the corresponding circularly polarized organic light-emitting diodes (CP-OLEDs) exhibit efficient CPEL signals with electroluminescence dissymmetry factor|gEL| of 3.4 × 10-3 and high maximum external quantum efficiency (EQEmax) of 17.8%. Notably, considering both EQEmax and |gEL| comprehensively, the device performance of R-PAC and S-PAC is the best among all the reported CP-OLEDs with chiral conjugated polymers as emitters. This work provides a facile approach to constructing chiral conjugated TADF polymers and discloses the potential of axially chiral conjugated luminescent skeletons in architecting high-performance CP-OLEDs.

Color‐Tunable TADF Conjugated Polymers Toward Voltage‐Regulating White OLEDs for Intelligent Lighting

AbstractColor‐tunable luminescent polymers are highly desirable materials for meeting the daily needs of humans in terms of circadian rhythm, yet they are rarely reported. Herein, a feasible approach is developed for color‐tunable thermally activated delayed fluorescence (TADF) conjugated polymers by incorporating green and red TADF units into blue fluorescent polymeric backbone. The solution‐processed OLEDs based on the synthesized polymers exhibit high‐efficiency red emission with a maximum external quantum efficiency (EQEmax) of 26.1% under low voltage (6 V). With a slight increase in voltage, warm white emission with EQEmax of 23.9%, color rendering index (CRI) of 76 and low corelated color temperature (CCT) of 1700 K is obtained. These results represent the highest efficiencies among the red and warm white OLEDs based on TADF polymers, respectively. Intriguingly, under higher voltage (10 V), warm white emission with high CRI of 87, CCT of 3216 K, and luminance of over 5300 cd m−2 is accessible. This work provides a promising approach for intelligent lighting by voltage‐regulating white OLEDs.

Cell-penetrating peptides noncovalently modified red phosphorescent nanoparticles for high-efficiency imaging.

The application of long-lived phosphorescence probes in time-resolved luminescence imaging is limited by their low quantum yield in aqueous solutions. However, sensitization of thermally activated delayed fluorescence (TADF) materials can compensate for this limitation while addressing the issue of insufficient proportion of their own long lifetime. In this study, we utilized the characteristics of phosphorescence and TADF materials simultaneously by doping the receptor iridium complex PMD-Ir into the donor TADF polymer PCzDP-20 through donor-receptor doping method, and successfully prepared highly efficient red phosphorescent nanoparticles. The quantum yield of the nanoparticles obtained by this method reaches up to 30%, and the luminescence lifetime can reach several thousand nanoseconds. Additionally, due to the low concentration doping of PMD-Ir, the risk of transition metal toxicity is greatly reduced. Furthermore, we used non-covalent modification with amphiphilic cell-penetrating peptides (CPPs) to increase the cell membrane permeability of the nanoparticles. The CPPs modified nanoparticles achieve in vivo confocal imaging of zebrafish and intracellular time-resolved imaging by its significantly improved bioimaging capabilities. The functional nanoparticles designing method fully utilizes the characteristics of PMD-Ir, PCzDP-20, and CPPs, solving the problems of low quantum yield and poor membrane permeability of Ir-complex nanoparticles. This will greatly promote the development of time-resolved luminescence imaging.

Adjusting the Electron-Withdrawing Ability of Acceptors in Thermally Activated Delayed Fluorescence Conjugated Polymers for High-Performance OLEDs.

Constructing high-performance solution-processed organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) conjugated polymers remains a challenging issue. The electron-withdrawing ability of acceptors in TADF units significantly affects the TADF properties of the conjugated polymers. Herein, we have designed three TADF conjugated polymers, in which phenoxazine donors and anthracen-9(10H)-one acceptors are incorporated into the polymeric backbones and side chains, respectively, and the carbazole derivative is copolymerized as the host. By incorporating different heteroatoms, such as nitrogen, oxygen, or sulfur, with slightly different electronegativities into anthracen-9(10H)-one, the effect of the electron-withdrawing ability of the acceptor on the performance of conjugated TADF polymer-based OLEDs is thus systematically studied. It is found that the introduction of a nitrogen atom can enhance the spin-orbital coupling and RISC process due to the modulated energy levels and nature of the excited states. As a result, the solution-processed OLEDs based on the prepared polymer p-PXZ-XN display an excellent comprehensive performance with an EQEmax of 17.6%, a low turn-on voltage of 2.8 V, and a maximum brightness of 14750 cd m-2. Notably, the efficiency roll-off is quite low, maintaining 15.1% at 1000 cd m-2, 12.1% at 3000 cd m-2, and 6.1% at 10000 cd m-2, which ranks in the first tier among the reported TADF conjugated polymers. This work provides a guideline for constructing high-efficiency TADF polymers.

High‐Efficiency TADF Polymers with a Spatially Confined Conjugated Backbone Enable Solution‐Processable Blue OLEDs Realizing over 20% EQE

AbstractThe concept of thermally activated delayed fluorescence (TADF) conjugated polymers has the advantage of enabling solution‐processable devices and harnessing singlet and triplet excitons simultaneously, whereas the resultant redshift of emission spectra and inevitable drop‐off of triplet excited states are detrimental to exploring high‐efficiency blue conjugated polymeric emitters. Herein, a feasible molecular design strategy is proposed by combining a spatially confined conjugated backbone and a TADF moiety isolated by a saturated spiro spacer to enable blue emission in newly designed partly conjugated TADF polymers, simultaneously achieving an excellent photoluminescence quantum yield of over 80% and a relatively high reverse intersystem crossing rate of 4.2 × 105 s−1. Endowed by superior photophysical properties and balanced carrier mobility, a maximum external quantum efficiency of 20.5% is achieved with emission at 486 nm and Commission Internationalede l'Eclairage coordinates of (0.18, 0.31), which is so far the highest efficiency for solution‐processed blue TADF polymer light‐emitting diodes.

Recent progress in thermally activated delayed fluorescence dendrimers for solution‐processed organic light‐emitting diodes

AbstractDuring the past decade, the discovery of thermally activated delayed fluorescence (TADF) materials has significantly boosted the development of organic light‐emitting diode (OLED) technology. Compared with small‐molecule and polymeric TADF materials, TADF dendrimers have emerged as promising emitters for solution‐processed OLEDs because they have the integrated advantages of TADF small molecules and polymers in achieving high efficiency, excellent solution processability, and precise molecular structures. In recent years, TADF dendrimers have experienced important advances in molecular design, mechanism exploration and device performance. Herein, we present a comprehensive review of solution‐processable TADF dendrimers, mainly focusing on their molecular design principles and structure–property correlations. The advanced device performances of these dendrimers are also summarized. Finally, we proposed the prospects and challenges on the development of TADF dendrimers.

Molecular design strategy for through-space charge transfer blue polyimides with rigid non-conjugated backbone and the role of alicyclic imide linker

The exploration of blue polymeric emitters plays an important role in the application of solution-processed polymer light-emitting diodes (PLEDs) in full color display. But the development of highly efficient blue thermally activated delayed fluorescence (TADF) polymers is still extremely challenging. Here, a series of rigid non-conjugated polyimide (PI)-based blue thermally activated delayed fluorescence (TADF) polymers with through-space charge transfer effect were firstly reported based on our novel design strategy. Among them, TRZ-oC-ph (λem = 497 nm) with a typical face-to-face structure as TADF unit was introduced into a polyimide backbone using twisted alicyclic imide as Linker unit. The twisted Linker unit with different structures can effectively regulate the d-spacing between donor and acceptor in the TADF unit, thus obtaining a series of blue TADF polyimides with λem at 485 nm, and high thermal stability (Tg > 277 °C, Td > 477 °C). Remarkably, highly-efficient solution-processing blue polymer light emitting diodes (PLEDs) based on the above polyimides were successfully assembled, with a maximal external quantum efficiency of 23.2%. Such outstanding efficiency is amongst the state-of-the-art performance of non-conjugated PLEDs. It confirms the effectiveness of the structural design strategy, and provides useful and valuable guidance for the development of highly-efficient blue TADF polymer materials and PLEDs.

Unveiling the In Situ and Solvent Polymerization Engineering for Highly Efficient and Flexible Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes.

Thermally activated delayed fluorescence (TADF) polymer has great potential for the construction of flexible solution-processed organic light-emitting diodes (OLEDs). However, the relationship between polymerization engineering and device functions has rarely been reported. Here, two novel TADF polymers, P-Ph4CzCN and P-Ph5CzCN, with a small energy gap between the first excited singlet and triplet states (ΔEST; <0.16 eV) were newly developed by both solvent and in situ polymerization of a styrene component. Detailed device performance testing indicates that both polymerization strategies ensure that the TADF polymer achieves comparable high efficiencies in commonly rigid devices, and the maximum external quantum efficiencies (EQEmax) were 11.9%, 14.1%, and 16.2% for blue, green, and white OLEDs, respectively. Although in situ polymerization provides a simplified device fabrication process, which avoids the complicated synthesis and purification of the polymer, the inevitable high-temperature annealing makes it fail in a plastic substrate device. In contrast, P-Ph5CzCN achieved by solvent polymerization enables the successful fabrication of a flexible device on a poly(ethylene terephthalate) (PET) substrate, which was the first reported flexible OLED based on a TADF polymer. This work provides a strong guideline for the simple fabrication of TADF polymer devices and the application of TADF polymer materials in OLED flexible panels and flexible lighting.

Chiral TADF Polymers Realizing Highly‐Efficient Deep‐Red Circularly Polarized Electroluminescence Over 660 nm

AbstractCircularly polarized electroluminescence (CPEL) from thermally activated delayed fluorescence (TADF) emitters have been widely reported in recent years. However, a universal strategy for designing deep‐red chiral TADF emitters is still unexplored. Herein, to address the problem, a pair of chiral donor intermediates are designed and synthesized for preparing deep‐red chiral TADF emitters, and two novel chiral TADF polymers R‐P and S‐P are developed through this strategy. The polymers exhibit intense mirror‐image circularly polarized luminescence (CPL) signals in toluene dilute solution. Besides, a series of solution‐processed circularly polarized polymer light‐emitting diode (CP‐PLED) devices are fabricated with R‐P and S‐P. The corresponding devices achieve persuasive performance with external quantum efficiencies (EQEs) of 6.2% and 5.8% at the wavelength of 662 nm, respectively. Besides, mirror‐image CPEL signals are detected with electroluminescence dissymmetry factors (gEL) of +1.6 × 10−3 and −1.7 × 10−3 from the corresponding CP‐PLED devices, respectively.

Polymerized Thermally Activated Delayed‐Fluorescence Small Molecules: Long‐Axis Polymerization Leads to a Nearly Concentration‐Independent Luminescence

AbstractNowadays numerous thermally activated delayed fluorescence (TADF) polymers have been developed for PLEDs to realize high device performance and tunable emission colors. However, they often possess a strong concentration dependence on their luminescence including aggregation‐caused quenching (ACQ) and aggregation‐induced emission (AIE). Herein, we first report a nearly concentration‐independent TADF polymer based on the strategy of polymerized TADF small molecules. It is found that when a donor‐acceptor‐donor (D‐A‐D) type TADF small molecule is polymerized through its long‐axis direction, the triplet state is distributed along the polymeric backbone to effectively suppress the unwanted concentration quenching. Unlike the short‐axis one with an ACQ effect, the photoluminescent quantum yield (PLQY) of the resultant long‐axis polymer remains almost unchanged with the increasing doping concentration. Accordingly, a promising external quantum efficiency (EQE) up to 20 % is successfully achieved in a whole doping control window of 5–100 wt. %.

Polymerized Thermally Activated Delayed-Fluorescence Small Molecules: Long-Axis Polymerization Leads to a Nearly Concentration-Independent Luminescence.

Nowadays numerous thermally activated delayed fluorescence (TADF) polymers have been developed for PLEDs to realize high device performance and tunable emission colors. However, they often possess a strong concentration dependence on their luminescence including aggregation-caused quenching (ACQ) and aggregation-induced emission (AIE). Herein, we first report a nearly concentration-independent TADF polymer based on the strategy of polymerized TADF small molecules. It is found that when a donor-acceptor-donor (D-A-D) type TADF small molecule is polymerized through its long-axis direction, the triplet state is distributed along the polymeric backbone to effectively suppress the unwanted concentration quenching. Unlike the short-axis one with an ACQ effect, the photoluminescent quantum yield (PLQY) of the resultant long-axis polymer remains almost unchanged with the increasing doping concentration. Accordingly, a promising external quantum efficiency (EQE) up to 20 % is successfully achieved in a whole doping control window of 5-100 wt. %.

Realizing External Quantum Efficiency over 25% with Low Efficiency Roll-Off in Polymer-Based Light-Emitting Diodes Synergistically Utilizing Intramolecular Sensitization and Bipolar Thermally Activated Delayed Fluorescence Monomer

Realizing External Quantum Efficiency over 25% with Low Efficiency Roll-Off in Polymer-Based Light-Emitting Diodes Synergistically Utilizing Intramolecular Sensitization and Bipolar Thermally Activated Delayed Fluorescence Monomer

Thermally Activated Delayed Fluorescence Polysiloxanes with Short Delay Fluorescence Lifetimes.

Blue-emitting thermally activated delayed fluorescence (TADF) polymers are still in demand for high-efficiency display materials. Through-space charge transfer (TSCT) strategy is promising for keeping color purity of blue-emitting polymers with nonconjugated main chains. It is, however, hard to synthesize copolymers with well-dispersed donors or acceptors utilizing traditional polyethylene backbones via radical polymerization. Herein, two series of blue-emitting polysiloxane with TADF properties, random and order-controlled copolysiloxanes, are successfully designed and synthesized and their photophysical properties are investigated and compared in detail. All of them display short prompt and delay fluorescence lifetimes and a very fast reverse intersystem crossing (RISC) rate of 107 s-1 . Compared with random copolysiloxanes, acceptors are well separated by donors for order-controlled copolysiloxanes, which exhibit the faster RISC processes and the higher photoluminescence quantum yield. Therefore, the order-controlled architecture provides a guide for improving light-emitting efficiency of TSCT-type TADF polymers.

Efficient narrowband red electroluminescence from a thermally activated delayed fluorescence polymer and quantum dot hybrid

Multiresonant effect is believed to be a promising molecular design rule to solve the bottleneck of wideband electroluminescence (EL) for thermally activated delayed fluorescence (TADF) materials. However, the realization of narrowband red EL still remains a big challenge due to the limited intramolecular charge transfer. Here, a hybrid emitting layer (EML) composed of a TADF polymer and red quantum dots (R-QDs) has been proposed for narrowband EL in red region. Such a hybrid EML can ensure efficient Förster resonance energy transfer and charge trapping to some degree, improve photoluminescent quantum yields, harvest the generated triplet excitons, and favor hole injection/transporting to simply device structure free of hole-transporting layer. As a consequence, the wideband TADF with a full-width at half-maximum (FWHM) of 87 nm is successfully converted to a narrowband red emission with a FWHM of 29 nm. Also, the corresponding device achieves a state-of-art external quantum efficiency as high as 14.9% (16.7 cd A-1, 16.3 lm W-1) together with Commission Internationale de l’Eclairage coordinates of (0.67, 0.32). The results clearly indicate the great potential of the hybrid EML for narrowband red EL.

Creation of efficient solution-processed OLEDs via a strategy of the host-guest system constructing with two small cross-linkable TADF molecules

Based on the strategy of using small cross-linkable thermally activated delayed fluorescence (TADF) molecules act as the host or the guest, a highly efficient TADF polymer is successfully constructed to develop efficient solution-processed organic light-emitting diodes (OLEDs). In this paper, two small cross-linkable TADF monomers, namely DV-2CzBN and DV-2CzTBN, are designed and synthesized. Simultaneously, small molecular TADF monomers are linked together using two vinylbenzyl linkers to form polymers. Systematic studies demonstrate that the polymers based on the host-guest TADF system can maintain the intrinsic TADF nature unchanged, and also effectively inhibit the triplet excitons quenching. Encouragingly a solution-processed OLED based on the host-guest system TADF polymer showed an excellent device performance with the maximum current efficiency (CE) of 25.1 cd A −1 , the power efficiency (PE) of 19.6 lm W −1 and external quantum efficiency (EQE) of 11.4%, respectively, which are two times higher in device efficiency than the corresponding device with the polymers based on the single-molecule system. Here, a new series of non-conjugated polymers were designed and synthesized based on the strategy of the host-guest system constructing with two small cross-linkable TADF molecules. • A new series of TADF polymers have been synthesized based on two small cross-linkable TADF molecules. • The TADF characteristics of small cross-linkable molecule are perfectly inherited by these VBN-based polymers. • The TADF behaviors can be improved by managing the content of DV-2CzTBN unit in the polymers. • The VBN10-based device can afford a high TADF OLED by solution process with EQE of 11.4%.

Constructing high-performance TADF polymers from non-TADF monomers: a computational investigation.

Thermally activated delayed fluorescence (TADF) polymers excelling in simple, low-cost and large-area solution process ability have attracted tremendous attention recently, but it remains a great challenge for the design of such materials due to the lack of reliable molecular construction guidelines. Here we perform a systematic computational investigation on the construction of TADF polymers from non-TADF monomers to elucidate the effects of polymerization sites, substituent positions and substituent types. The results indicate that the polymerization of 3,6-carbazole-based monomers with different substituents is efficient to build TADF polymers due to their facile π-conjugation extendability. Especially, polymers with para-phenyl-substituted monomers are promising in light of their separated frontier molecular orbitals for small ΔEST with favorable energy levels, bipolar charge transport properties and relatively strong absorption/emission intensity, which should be highly attractive for experimental investigations. These findings and insights are important in revealing the structure-property relation of TADF polymers made from non-TADF monomers with important clues for understanding the construction mechanism and molecular design principles of TADF polymers.

Chiral TADF‐Active Polymers for High‐Efficiency Circularly Polarized Organic Light‐Emitting Diodes

AbstractA strategy of chiral donor–acceptor copolymerization is proposed to develop chiral nonconjugated polymers with thermally activated delayed fluorescence (TADF). Based on this strategy, two pairs of chiral polymers (R,R)‐/(S,S)‐pTpAcDPS and (R,R)‐/(S,S)‐pTpAcBP were synthesized. The alternating copolymerization of the chiral donors and acceptors could effectively separate the frontier molecular orbitals, which made the polymers show small ΔEST of 0.01–0.03 eV and efficient TADF properties. Moreover, the polymers also showed the quantum yield of up to 92 % and the circularly polarized luminescence. The solution‐processed circularly polarized organic light‐emitting diodes showed circularly polarized electroluminescence signals with high external quantum efficiencies of up to 22.1 % and maximum luminance of up to 34350 cd m−2. This is the first report of CP‐OLEDs based on chiral TADF polymer, which provides a useful and valuable guidance for the development of high‐efficiency CPEL polymers.

Chiral TADF-Active Polymers for High-Efficiency Circularly Polarized Organic Light-Emitting Diodes.

A strategy of chiral donor-acceptor copolymerization is proposed to develop chiral nonconjugated polymers with thermally activated delayed fluorescence (TADF). Based on this strategy, two pairs of chiral polymers (R,R)-/(S,S)-pTpAcDPS and (R,R)-/(S,S)-pTpAcBP were synthesized. The alternating copolymerization of the chiral donors and acceptors could effectively separate the frontier molecular orbitals, which made the polymers show small ΔEST of 0.01-0.03 eV and efficient TADF properties. Moreover, the polymers also showed the quantum yield of up to 92 % and the circularly polarized luminescence. The solution-processed circularly polarized organic light-emitting diodes showed circularly polarized electroluminescence signals with high external quantum efficiencies of up to 22.1 % and maximum luminance of up to 34350 cd m-2 . This is the first report of CP-OLEDs based on chiral TADF polymer, which provides a useful and valuable guidance for the development of high-efficiency CPEL polymers.

Polymer Electrochemiluminescence Featuring Thermally Activated Delayed Fluorescence.

Electrochemiluminescence (ECL) based on conjugated polymers or oligomers is persistently being pursued owing to its huge application scope ranging from ultra-sensitive bioanalysis to ultra-resolution imaging and spectroscopy. Because of the theoretical limit in radiative exciton generation yield (typically ∼25 %) of those polymers or oligomers, the corresponding ECL efficiency is still limited, which hampers its ECL performance and its related applications. Herein, we report ECL based on a thermally activated delayed fluorescence (TADF) polymer scaffold, which is characteristic of all-exciton harvesting in the ECL process, and thus potentially capable of achieving ∼100 % ECL efficiency. These desired properties of the TADF polymer ECL is attributed to a fast and efficient up-conversion process from non-radiative triplet to radiative singlet states under thermal activation, which is absent in conventional fluorescent polymers/oligomers, such as F8BT. In this study, various ECL modes, including annihilation or co-reactant mode using TPrA or S2 O82- as co-reactant, are confirmed for our model TADF polymer ECL system, which was different from fluorescent polymer ECL counterpart. Furthermore, solid-state ECL sensing on L-cysteine (an important marker of disease) is also evaluated by using the model TADF polymer. Ultralow detection limit in combination with high sensitivity and good specificity are achieved for this model system, indicative of a high potential of the TADF polymer scaffold for applications in the broad field of ECL.